The present invention relates to an ignition timing control method of an internal combustion engine of the spark ignition type.
When the ignition timing of an engine is to be controlled based upon a spark advancing value which is digitally calculated relying upon various signals which indicate the operating condition of the engine, for example, relying upon rotational speed signals which indicate the rotational speed of the engine, intake amount signals which indicate the amount of the air sucked into the engine, and the like, it is accepted practice to calculate, from the spark advancing value, an angle between a reference portion of and crankshaft to the position of the crank shaft corresponding to the ignition timing, or to calculate a value which corresponds to the period of time required by the crankshaft to rotate from a reference crank angle to a crank angle corresponding to the ignition timing. The value obtained by the calculation is reset, for example, in a downcounter and is counted down in response to clock signals which are produced after every crank angle increment of a predetermined size or after every time interval of a predetermined length. The operation of the count down is started from the moment at which the crankshaft reaches the position of the above-mentioned reference angle. Therefore, the moment at which the counted value of the downcounter becomes zero is recognized as an ignition timing.
Among the methods of controlling the ignition timing of this type, the method which counts down at fixed time intervals controls the ignition timing irrespective of the operation conditions of the engine, particularly irrespective of the rotational speed of the engine, after the calculation operation has finished and the downcounter has started the counting operation. Therefore, if the rotational speed of the engine is quickly changed during this period, the ignition timing no longer responds to the change in the rotational speed.